Add contributing guidelines

[cjrace]

Would be helpful to have a dedicated contributing page that pulls from our knowledge and the R Package (2e) book by Hadley Wickham and Jennifer Bryan that lowers the bar for more DfE analysts to be able to contribute to this.

Some quick initial thoughts:

• Standard workflows for running the package locally with devtools and usethis
• Detailing the purpose for the code
• Adding suitable documentation using roxygen
• Adding suitable test coverage using testthat
• Ensuring that code meets the tidyverse styling convention
• Get approving review from at least one of the maintainers

[cjrace]

Have started a dedicated page, though more details still needed:

• [x] Basic walkthrough of adding a new function
• [x] Who to reach out to for support
• [x] More detail on package versioning
• [x] More detail on commit hooks
• [x] Detail on code coverage setup